High attenuation coaxial cable



Dec. 16, 1952 s. J. ROSCH HIGH ATTENUATION COAXIAL. CABLE Filed Sept.21, 1946 INVENTOR SAMUEL J. R H

[ JLQW Wn' uE/ ATTORNEY Patented Dec. 16, 1952 HIGH ATTENUATION COAXIALCABLE Samuel J. Rosch, Yonkers, N. Y., assignor to Anaconda Wire andCable Company, a corporation of Delaware Application September 21, 1946,Serial No. 698,498

2 Claims. 1

This invention relates to electric cables, and more particularly tohigh-attenuation coaxial cables for conducting electric currents atfrequencies above 100 megacycles. The new cable is of conventionalcoaxial construction in that it comprises a central metallic conductor,a surrounding layer of dielectric material, and an outer cylindricalmetallic conductor. The characteristic feature of the new cable is theprovision of a separat thin-walled cylindrical conductor 0.001 to 0.010inch in thickness having high electrical resistance positioned betweenthe outer conductor and the dielectric. It is this separate highresistance conductor that imparts to the cable its high-attenuationcharacteristics.

Coaxial cables find important uses in making connections betweenhigh-frequency electronic devices, because such cable is particularlyefflcient as a conductor of high-frequency electrical energy-that is, itconducts such energy with minimum power attenuation. Occasions arise,however, when a coaxial cable having high-attenuation characteristics isrequired. For example, it is often desirable to tap off a fraction ofthe power being conducted by a main highfrequency circuit withoutintroducing disturbing influences into the main circuit. Iflow-attenuation cable is used for this purpose, the amount of powerdrained from the main circuit may seriously disturb proper operation ofthe main circuit. In such case, therefore, a high-attenuation coaxialcable is ordinarily preferred.

As another example, it is often necessary to connect two mismatchedhigh-frequency electronic devices by means of a low-attenuation coaxialcable. In such case the mismatching of the two devices may causestanding waves to develop in the cable connecting them, with the resultthat only a small fraction of the power fed in at one end of the cableisusefully delivered at the other end. It is often possible to correctthis condition by properly connecting a matching terminating section ofhigh-attenuation coaxial cable to the receiving device supplied withpower by the low-attenuation cable. The terminating section ofhigh-attenuation coaxial cable dissipates the power that would otherwisebe reflected back from the mismatched power-receiving device to createstanding waves in the connecting cable.

High-attenuation coaxial cables of known attenuation characteristics arealso useful for measuring the attenuation of other devices (such'as'high-frequency power lines) having the same characteristic impedanceas the cable used for 2 making the measurement. Such cable also may beused for connecting electric measuring instruments into high-frequencycircuits, especially where the instruments used have scale readings toolow to permit being connected directly into the circuit bylow-attenuation connections, or where the instrument would createexcessive disturbances if thus connected directly to the circuitto bemeasured. Multiplying the instrument reading by, or adding to it, anappropriate conversion factor derived from the known attenuationcharacteristics of the high-attenuation cable by which it is connectedin the circuit then gives a proper indication of the quantity desired tobe measured in the main circuit.

The standard high-attenuation coaxial cabl heretofore known and employedhas been of conventional coaxial cable construction, except that thecentral conductor has been composed of a metal having rather highresistance. Nichrome wire (composed of an alloy of nickel and chromium)has been used almost exclusively as the central conductor in suchcables. High-attenuation coaxial cables of this heretofore knownconstruction have a number of serious drawbacks; The Nichrome generallyused for the central conductor is difiicult to solder, and accordinglyit is not easy to make good electrical connections to it. The resistanceof Nichrome" varies appreciably with changes in temperature, andconsequently the attenuation characteristics of coaxial cables employinga Nichrome central conductor vary markedly with temperature. Since theuse of high-attenuation cables involves dissipation of power, and sincethe power is invariably dissipated as heat, the dependence of theattenuation characteristics of Nichrome-core coaxial cable on temperaturis particularly serious. Most of the heat generated in Nichrome-corehigh-attenuation coaxial cables is produced within the Nichromecore wireitself, and must be dissipated by conduction through the dielectricbetween the two conductors and through the outer insulating jacket (ifany) to the atmosphere. The insulating materials most commonly used formaking the dielectric and the outer insulating jacket are poorconductors of heat, and moreover become damaged if heated excessively.Consequently the heat (power) dissipation rating of such cables mustbelow, and long lengths of quite large cable of this character must beemployed if any very large amount of power is to be dissipated.

The present invention provides an improved high-attenuation coaxialcable that for themost.

part is free of the objections inherent in the Nichrome-core type ofhigh-attenuation cable heretofore commonly employed, and that at thesame time possesses distinctive advantages of its own. Basically, thenew cable comprises a central metallic conductor, a layer of dielectricmaterial surrounding the central conductor, and an outer metallicconductor surrounding the dielectric, and is characterized by theprovision of a conductor in the form of a thin-walled cylinder havinghigh electrical resistance positioned between the outer conductor andthe dielectric. The outer metallic conductor immediately surrounds thehigh-resistance conducting element,

and is in contact with it over substantially its entire outer surface.Consequently these two elements together constitute the complete outerconductor of the cable.

The high-resistance conductor advantageously comprises a non-conductingmaterial made conductive by incorporation therein of finely-divided.carbon. Paper impregnated with a finelydivided carbon such as carbonblack is particularly satisfactory. Such paper in the form of tape maybe wound helically about the dielectric material prior to applying theouter conductor, to form a thin-walled conductive cylinder underlyingthe outer metallic conductor. In place of carbon-impregnated paper,however, the high-resistance conductor may comprise some other materialsuch as rubber, polyethylene resin, polyvinyl chloride resin, or otherplastic composition made conductive by being impregnated withfinelydivided carbon. The resulting conductive plastic composition maybe extruded or otherwise formed into a thin-walled cylinder surroundingthe dielectric and underlying the outer metallic conductor, orsurrounding the inner conductor and underlying the dielectric.

If desired, the new cable may be provided with an outer insulatingjacket of any suitable material.

'The invention is described in greater detail below in connection withthe accompanying draw ing, the single figure of which is a perspectiveshowing the construction of a particularly advantageous embodiment ofthe new cable.

The cable shown in the drawing comprises a central metallic conductor Isurrounded by a layer of dielectric material 2. Wrapped helically aboutthe dielectric is a tape 3 of paper impregnated with carbon black torender it conductive. The helically wrapped tape forms a thin-walledconducting cylinder having high. electrical resistance: Generally it ispreferable to wrap the paper tape with edges abutting, but if desiredthe' edges may overlap, or may even be separated somewhat. The type ofwrap used maybe chosen for the purpose of varying the electricalresistance somewhat. Alternatively to helically wrapping the paper aboutthev dielectric, it may be tubed longitudinally thereabout. A braid 4 ofmetallic wires is applied immediately over the carbon black paper tape 3and in contact therewith. The braid and the carbon-black paper togetherform the outer conductor of the cable, and are held in. coaxial relationwith the central conductor by the intervening layer of dielectricmaterial. An outer insulating jacket 5 completes the cable assembly.

Both the central. conductor l and the outer metallic braid 4 are ofcopper or other good conducting metal. The dielectric 2 thatseparatesthe inner and outer conductors and maintains them in their coaxialrelationship advantageously is tivity of. the carbon-black paper.

a material (such as a polyethylene composition) which possesses gooddielectric properties at high frequencies. The outer insulating jacket 5may be composed of a rubber composition, or of some other plasticcomposition such as a composition of a polyethylene or polyvinylchloride. Provision of such an outer insulating jacket often isdesirable but is not always necessary.

The new cable conducts direct current and lowfrequency alternatingcurrent in substantially the same manner as conventional coaxial cablenot having the wrapping of carbon-impregnated paper tape 3. Inconducting such currents, the outer conductor may be considered ascomposed of a low-resistance conducting element (the metallic braid 4)and a high-resistance conducting element (the carbon impregnated paper3) connected in parallel. In such case most of the current carried bythe outer conductor flows through the outer braid 4. In conductingcurrents at high frequencies, however, sufficient loss is developedthrough the impregnated paper 3 to result in ap preciable attenuation.As the frequency is increased, slrin erlects become more and morepronounced, so that an increasing amount of current is shifted from themetallic outer braid to the inner spirally wrapped conductive element.Since this medium possesses high electrical resistance, substantialpower loss and high attenuation. resultin the range of high frequenciesatwhich the new cable is designed to operate.

It is possible to control the frequency at which maximum attenuationcharacteristics first develop in the cable by controlling the thicknessof the carbon black paper. The thicker the carbon black paper wrapping,the lower will be the frequency at which high attenuation begins toappear. For most uses, the thickness of the carbon-black paper or otherhigh. resistance conductive element is advantageously between about0.001 inch and 0.010 inch, although thicknesses above or below theselimits are sometimes desirable. Further, since the extent to which thecable attenuates depends upon the resistivity of the conductingpathairorded by the carbonblack paper, it is possible to control (withinlimits) the amount of attenuation that occurs in the cable bycontrolling the electrical resis- This in turn may be controlled readilyby controlling the amount of carbon black impregnated into the paper-The extent to which the cable attenuates at high frequencies increases(within limits) as the amount of carbon impregnated in the paper (andhence the conductivity of the paper) de creases.

The following tabulation shows the attenu ating characteristics ofcableconstructed as described above, in comparison with other types ofcoaxial cables. In the following tabulation, cable A was a standard.RG8/U coaxial cable havinga copper central conductor and a copper outerconductor. Cable B was of the same construction as cable A., except thatcarbon-black paper was wrapped helically about the dielectric separatingthe central and outermetallic conductors- Cable 0 was a standard typeRG-Zl/U coaxial cable having. a Nichrome wire central conductor and acopper outer conductor; Gable D was identical with cable C, except thatthe central conductor was copper, and paper impregnated with carbonblack was wrapped hell cally about the dielectric immediately beneaththe outer metallic conductor. In both cables B and D the carbon-blackpaper on the cable possessed a D. C. resistance of 3.80 megohms perfoot. The figures in the body of the table give the attenuation of thecables in decibels per 100 feet at 26 C.

Comparing the attenuation characteristics of cables A and B in the abovetable, it will be observed that the new cable (cable B) hassubstantially greater attenuating characteristics than standard copperconductor coaxial cable. At the lower frequency the new cable attenuatesapproximately three times as much per unit of length as does the regularcable, and at a frequency of 3000 megacycles it attenuates almost fourtimes as much. Comparing the new cable with the heretofore knownNichrome-core attenuating cable, it will be noted that at the lowerfrequencies the new cable (cable D) does not attenuate quite so much asthe Nichrome-core cable (cable C). At the high frequency of 3000megacycles, however, the new cable has substantially greater attenuatingcharacteristics than the Nichrome"-core cable.

Other thin-walled cylindrical conductors having high electricalresistance may be used in the new cable in place of the paper tapeimpregnated with carbon black and wrapped helically about the dielectricas particularly described above and shown in the drawing. For example,rubber made conductive by being heavily loaded with finely-dividedcarbon may be extruded as a thin jacket about the dielectric. Similarly,other non-conducting plastics such as polyethylene compositions orpolyvinyl chloride compositions made conductive by incorporation thereinof finely-divided carbon may be extruded about the dielectric, or tapesor strips of such compositions may be wrapped about the dielectric.

The new cable possesses a number of advantageous characteristics notpossessed by the Nichrome-core high-attenuation coaxial cable,

heretofore frequently used. Not the least of these advantages is theability of the new cable to conduct direct current or low-frequencyalternating currents eificiently (i. e., without substantialattenuation). The new cable, therefore, may

be employed advantageously where it is desired to conduct such currentsover the same conductor in which high-frequency currents are to beattenuated. Thus the new cable may be used in some circuits as alow-pass filter.

Another advantage of the new cable when the high resistance elementsurrounds the dielectric is that the heat generated in consequence ofits attenuation characteristics is liberated for the most part in thehigh-resistance conductor immediately underlying a metallic conductor.This heat is readily and rapidly conducted through the outer metallicconductor, and its dissipation to the atmosphere is impeded only bywhatever outer insulating jacket is applied to the cable. Consequently,its heat dissipating capabilities are substantially greater than is thatof Nichromecore coaxial cable, in which the heat is generated inlthecentral conductor and must be conducted through the dielectric as wellas through an outer insulating jacket. The new cable, therefore, maysafely carry a higher heat-dissipation rating than the Nichrome-corecoaxial cable.

1 claim:

1. A high-attenuation coaxial cable for conducting electric currents atfrequencies above megacycles comprising a central metallic conductor, alayer of insulation having good dielectric properties at frequenciesabove 100 megacycles surrounding the central conductor, and an outermetallic conductor surrounding the dielectric, characterized in that atape of paper I about 0.001 to 0.010 inch in thickness impregnated withcarbon black is wrapped helically about the dielectric beneath the outermetallic conductor, whereby heat produced in consequence of powerdissipation during operation of the cable at a high frequency isdeveloped substantially entirely outside the high-frequency insulationwhere it is readily transferred to the surrounding atmosphere.

2. A high-attenuation coaxial cable for conducting electric currents atfrequencies above 100 megacycles comprising a central metallicconductor, a layer of insulation having good dielectric properties atfrequencies above 100 megacycles surrounding the central conductor,

v a tape of paper about 0.001 to 0.010 inch in thickness impregnatedwith carbon black wrapped helically about the dielectric and formingthereabout a cylindrical conductor having high electrical resistance,and an outer metallic conductor immediately surrounding and in contactwith the carbon-black-impregnated paper, whereby heat produced inconsequence of power dissipation during operation of the cable at a highfrequency is developed substantially entirely outside the high-frequencyinsulation where it is readily transferred to the surroundingatmosphere.

SAMUEL J. ROSCH.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS OTHER REFERENCES Principles of Radar, RadarSchool, M. I. T., McGraw-Hill Book Company, Inc., N. Y., 1946. (Page 8-5relied upon.)

